The present invention relates to a method for preparing the hard structures of teeth for the application of dental restorative materials.
The adhesion of the dental restorative materials to the hard structures of the tooth, mainly enamel and dentin, is of paramount importance for dental preservation techniques.
The use, for example, of compound resins for tooth reconstruction has significantly increased due to the extremely high esthetic requirements of today""s population. However, such resins have an unwanted secondary feature due to their polymerization, which is a significant contraction causing them to be detached from the tooth. For this reason or for obvious retention requirements in preparations which are not self-retaining, adhesive substances must be used, which allow a bond between the mineralized tissues of the teeth and the compound resins. Therefore, the adhesive should perform the essential function for the success of the reconstruction, guaranteeing a marginal seal for the restoration made of compound resin.
To increase the retentive capacity of the substrate, for many years the treatment of dentin and enamel with various acidic solutions (or gels) was recommended. Such a process is called acid-etching and has a variety of different functions, depending on the tissue to which it is applied. On enamel, acid-etching makes the surface more receptive to the above-mentioned adhesive, dissolving the mineralized surface layer and revealing a rough, uneven surface. This surface is characterized by alternating so-called xe2x80x9cenamel prismsxe2x80x9d, that is to say, crystalline structures with varying orientation which guarantee that the adhesive resin can penetrate in small grooves and form interdigitations which guarantee retention and sealing. In practice, the fluid resin of the adhesive fills the microporosities in the enamel, guaranteeing micro-mechanical retention which allows the seal. Moreover, acid-etching increases the wettability of the enamel and allows the adhesive an angle of contact optimum for complete integration.
Dentin adhesion presents numerous problems because it has a different structure from enamel. Dentin consists not only of a mineral (hydroxyapatite) like enamel (in which 97% is mineral substance), but also an organic substance, that is to say, collagen fibrils which do not easily attach to acrylic hydrophobic resins. Moreover, dentin has a high water content, a factor which makes the sticking process even more complex.
In particular, dentin is a tissue characterized by a plurality of tubules which join the inside of the tooth (pulp) and the peripheral dentin. Inside these tubules there are the so-called xe2x80x9codontoblastic processesxe2x80x9d, that is to say, the cellular extensions of pulp cells (odontoblasts) hydrated by the fluid from blood circulation. Each tubule is surrounded by a hood of hypermineralized dentin, called the peritubular dentin. The dentin between one tubule and another is called intra-tubular, is very fibrous and less mineralized. The tubules are arranged radially starting from the pulp and have a decreasing diameter, therefore, their density differs according to the dentinal zone in which they are located and, therefore, the adhesion substrate changes according to the zones.
When the dentin is treated at given affected zones by removing the layer of tissue affected by the disorder, for example it is treated with a manual or rotary instrument to remove the decayed process or to prepare a cavity, a layer of debris called the xe2x80x9csmear layerxe2x80x9d is produced, which occludes the orifices of the dentinal tubules and covers the intertubular dentin, reducing its permeability.
The application of an etching acid on the dentin previously treated, therefore, removes the layer of debris, re-opening the dentinal tubules and demineralizing the top layer of dentin, removing the mineral part and leaving the organic matrix.
The process of demineralizing the surface of the dentin takes place both at intertubular and peritubular level, although the degree of incisiveness varies given the different levels of mineralization of the two structures. In particular, the tubules at the surface on which the etching acid was passed widen to a funnel shape and over the first 4 or 5 microns of thickness the intra-tubular tissues are completely demineralized, leaving only collagen fibrils.
These collagen fibrils, when in their so-called wet state, are porous, whilst when they are dry they collapse upon themselves and are reduced to a compact tissue on which, in contrast to the porous tissue, it is extremely difficult for the adhesive substances to stick.
At this point, the main problem of adhesion on the dentin is succeeding in completely penetrating the demineralized layer and entering several microns into the tubules so as to guarantee an effective seal. If part of the organic matrix is not reached by the fluid resin of the adhesive and remains exposed, it is rapidly degraded by salivary enzymes, forming a marginal gap at the sides of the restorative work.
Various methods of correctly infiltrating the dentin have been put forward, mainly aiming to keep the collagen fibrils (organic matrix exposed by acid-etching) from collapsing and to allow the adhesive to pass to the deepest zones.
Adhesive substances have been used which consist mainly of an acid-etching solution (mainly phosphoric acid 30-40%) and a polymerizable solution of hydrophilic and hydrophobic monomers. When the acid is washed away from the surface of the enamel and dentin, the monomer solution is applied (even in several layers) to the etched surface of the tooth. The most modern adhesives use acetone or ethanol as a solvent. The function of the solvent is to remove and substitute the water from the demineralized dentinal matrix.
The adhesive molecules have two functional groups: one with a great affinity for the surface of the tooth (hydrophilic) and another with the resinous material (hydrophobic) which is put over the layer of adhesive. The hydrophilic monomer which constitutes the main part of the adhesives is able to create micro-mechanical retention by interpenetrating the collagen fibrils exposed by the acid-etching, forming a mixed structure of organic matrix, inorganic residues and resin called the hybrid layer.
Formation of the hybrid layer still appears essential in order to guarantee a good degree of sealing. Moreover, the adhesive which enters the tubules which are open and widened to a funnel shape by the acid-etching process guarantees retention by forming resin extensions.
The absence of a seal can allow the infiltration of bacteria which may cause secondary decay under the reconstruction, which leads to rapid failure of the therapy. Also, it has been noticed that most breaks and yielding occur at the hybrid layer level, since complete impregnation with the resin is not achieved.
More specifically, the generation of zones which are not infiltrated seems to be the result of a variety of factors, including, for example, an acid-etching step which is too aggressive and demineralizes the fibrils too deep, so that where they are not reached by the resin, the fibrils are completely dried out and collapse, forming a compact layer which is difficult to infiltrate and an irreversible denaturation and, therefore, modification of the collagen fibrils following acid-etching or drying or cutter overheating.
All of these factors contribute to the creation of a layer which is not completely impregnated and with various microporosities which allow the passage of enzymes and bacteria.
It is, therefore, evident how the weak point of the entire adhesive process is guaranteeing precision filling of the open tubules after the acid-etching and complete infiltration of the collagen fibril matrix for a correct seal.
The aim of the present invention is to overcome the above-mentioned disadvantages to guarantee complete adhesion of the adhesive substances on the hard structures of the tooth.
Accordingly, the present invention provides a method for preparing the hard structures of teeth for the application of dental restorative material, characterized in that it comprises steps of treating at least one affected zone of the hard structure of the tooth, removing the layer of tissue affected by the disorder, applying a layer of adhesive substance at least in the zone treated, subjecting at least the adhesive substance to the action of an electrical field, applying the restorative material at least on the zone covered by the adhesive substance. The present invention also relates to a device for preparing the hard structures of teeth for the application of dental restorative materials.
In accordance with the present invention, a device is provided which prepares the hard structures of teeth for the application of dental restorative materials, characterized in that it comprises at least one handpiece connected to a control power circuit for applying an electrical field to at least one layer of adhesive substance, applied at least to an affected zone of the hard structure of the tooth, previously treated to remove the layer of tissue affected by the disorder, so as to apply the restorative material at least on the zone covered by the layer of adhesive substance.